This invention relates generally to heat exchangers for rapidly cooling the contents of a high-temperature chamber. In particular, this invention relates to a heat exchanger movable between a first position external to the chamber during a high-temperature step of a process and a second position internal to the chamber during a cooling step of the process.
Nitriding is a process of enriching with nitrogen the surface layer of steel, resulting in the formation of a hardened surface on machine components with improved fatigue, wear and/or seizing resistance. In conventional gas nitriding, carried out in partially dissociated ammonia gas at 500-600xc2x0 C., the superficial nitrogen concentration cannot be controlled. The combined nitrogen and carbon concentration at the surface reaches 11.3%, bringing about the formation of a single phase zone consisting mainly of carbonitrides. Such high nitrogen concentrations in the superficial layer are undesirable as they are the cause of porosity, brittleness and spalling. In addition, conventional gas nitriding cannot easily be repeated to achieve consistent results.
Liliental, Morawski and Tymowski teach in a paper entitled, xe2x80x9cControlled Gas Nitriding xe2x80x94The Modern Surface Treatment for the Automotive Industryxe2x80x9d published in the proceedings of the ASM conference on automotive heat treatment, Puerto Vallarta, Mexico, July 1998, a controlled atmosphere gas nitriding process, which process retains the major advantages of gas nitriding, including negligible dimensional changes and relatively low treatment temperature. Specifically, the nitriding potential of the furnace atmosphere, expressed as the ratio of ammonia and hydrogen partial pressures, is controlled. Further, a controlled rate of cooling after treatment can inhibit the formation of undesirable nitrides or carbonitrides in the compound and diffusion zones of the nitrided layer, thus improving the properties of the case.
Unfortunately, the maximum cooling rate that is attainable using a prior art furnace often is insufficient to rapidly cool the components while simultaneously maintaining a constant nitriding potential of the furnace atmosphere. As a result, the case hardened components may not develop the expected properties and may even be at risk of failing prematurely. More specifically, the toxic atmosphere of partially dissociated ammonia cannot easily be vented to outside of the furnace without first being neutralized using special equipment. Venting of the atmosphere creates an additional problem in that the cooling gas supplied to replace the vented atmosphere must have an identical composition, thus increasing the operating costs associated with acquisition and disposal of toxic chemical substances.
In U.S. Pat. No. 5,871,806, issued Feb. 16, 1999 in the name of Shoga et al., disclosed is a method and apparatus utilizing a cooling gas in order to cool an object in a cooling zone of a heat-treating apparatus. Shoga et al. teach a heat-treating apparatus comprising a heat-treating zone for heat-treating an object in a controlled atmosphere and a cooling zone for cooling the heat-treated object using a cooling gas, wherein the heat-treating zone and the cooling zone communicate through a door. Unfortunately, such an apparatus is poorly suited for batch-processing operations, for example using a pit furnace to case harden large loads of metal components.
It is a disadvantage of the prior art systems that nitrided components are transferred to a second other chamber subsequent to high-temperature treatment for cooling under a separate atmosphere. Often, undesirable nitride precipitates develop at component surfaces that are cooled under an improperly controlled atmosphere.
It would be advantageous to provide a cooling apparatus for use with a nitriding furnace, the cooling apparatus capable of cooling rapidly the contents of the furnace while other than affecting the chemical composition of an atmosphere contained by the furnace. A furnace equipped with such a cooling apparatus would be capable of producing high-quality nitrided components while avoiding undesirable porosity, brittleness and spalling.
In an attempt to overcome these and other limitations of the prior art it is an object of the instant invention to provide an apparatus for cooling rapidly the contents of a furnace.
In an attempt to overcome these and other limitations of the prior art it is another object of the instant invention to provide an apparatus for cooling rapidly the contents of a furnace while other than affecting the chemical composition of an atmosphere contained therein.
In an attempt to overcome these and other limitations of the prior art it is yet another object of the instant invention to provide an apparatus for cooling rapidly the contents of a furnace operating in a batch mode.
In accordance with the present invention there is provided a method for cooling a workpiece within a high-temperature chamber comprising the steps of: heating the workpiece; and, moving a moveable heat exchanger into the high-temperature chamber, such that a cooling rate of the workpiece is achieved that is relatively faster than a cooling rate obtained absent the moveable heat exchanger.
According to a further aspect of the invention there is provided an apparatus for cooling a workpiece within a high-temperature chamber comprising: a heat exchanger moveable between a first position in which the heat exchanger is thermally isolated from a gas atmosphere contained within the high-temperature chamber and a second position in which the heat exchanger is in thermal communication with the gas atmosphere; an actuator operatively coupled to the heat exchanger for moving selectively the heat exchanger between the first position and second position through a port of the high-temperature chamber; and, a seal disposed between opposing surfaces of the heat exchanger and the port for substantially containing the gas atmosphere within the high temperature chamber.
According to another aspect of the invention there is provided a heat treatment system comprising: a moveable heat exchanger; a high-temperature chamber having a port through a wall surface thereof for sealingly engaging an opposing surface of the heat exchanger and for permitting at least a portion of the heat exchanger to be removeably inserted within the high-temperature chamber, and having at least an orifice in communication with an atmosphere control system for providing a gas atmosphere with a predetermined composition; an actuator operatively coupled to the heat exchanger for relatively moving the heat exchanger to the high-temperature chamber to selectively insert the at least a portion of the heat exchanger through the port; a process controller in electrical communication with the actuator for providing to the actuator a first control signal for controlling the movement of the heat exchanger, wherein, in use, the heat exchanger is in thermal communication with the gas atmosphere when inserted in the high-temperature chamber and thermally isolated from the gas atmosphere when removed from the high-temperature chamber.